If you want to turn it into a science experience, you could use walnuts, beets, onion skins, tea, and things like that to dye them. But for vivid, lasting colors, I'd use something like dyes from Dharma Trading Co.

I used food coloring and they turned out great. You get a lot more flexibility with color that way, too -- I dyed two different shades of green, for instance. I just put enough into the water that it looked like the color I wanted, then went with it. The silks didn't come out that exact shade, but close enough, and they were even more gorgeous than I expected.

With food coloring, do you have to do any sort of acid bath to "fix" the dye so it doesn't run?

I soaked the silks in hot water and a couple glugs of white vinegar beforehand to make sure they were thoroughly saturated (5-10 minutes?). I also put a few glugs of vinegar in the water + food coloring.

Real exact, huh? I was so worried for my first one because I couldn't find specific instructions online. I finally got up the nerve to just make it up as I went (after having read a lot of instructions online that were close to my situation, but not exact) .... and it went great! I was a lot looser from then on out, and they still came out great!

I think I put in 1/4 cup vinegar in my stockpots of hot water + pigment when I dye silks. Any pigment will do, food coloring, or professional dyes are an option, or plant materials as mentioned above (many many things will give great colors, often way more pretty than chemical dyes). Turmeric will very easily give you a golden yellow (heck it stains everything without trying).

I did the food coloring with vinegar... I'm sorry I don't remember exactly but there are probably tutorials online somewhere! I had to out in LOTS of blue dye to dye my HUGE (several feet long) playsilk, but it worked great also for the smaller ones-- we made yellow, green, etc. And I had the old artificial food dye just sitting around, since we use food-based ones these days It really didn't take much!

This is great, thanks! Now how many should I make? I think I'll buy 35x35's from Dharma. We can make 5 - red, yellow, green, blue, pink. And maybe also leave one white? And should I make a couple smaller ones, like scarf size? Or are the 35's good enough? How many do you have?

Well heck if you are going to order from Dharma anyways its totally worth it to get a few of their silk specific dyes. I bought the three primary colors and dyed a set of playsilks from that. They turned out so great! The colors are super vibrant. I just followed the instructions on the Dharma website.

How much food coloring and vinegar do I need? I have 4 little bottles of food coloring and they're half full, and I have a half a bottle of vinegar. Should I buy more? I'll be dying 6 35x35's, 4 21x21's, and a bunch of skinny long ones.

You'll almost certainly need more, although it depends on how bright you want your colors. You've at least got enough to get started -- I'd suggest doing your first one or two, seeing how that goes, and then buying more supplies when you see how much you'll need.